1. Technical Field
The invention relates to resistor materials that are particularly sensitive to changes in temperature and find use in thermistors, bolometers, infrared detectors and the like.
2. Background of the Invention
In many applications, resistors are preferably made from materials whose resistivity is constant with temperature. However, these always exhibit a change in resistance with temperature. Quite often, this change is sensibly linear and is characterized by a sensibly constant temperature coefficient of resistance (TCR) which is numerically equal to the fractional change in resistance, dR, with change in temperature, dT, and given by (1/R) (dR/dT). (TCR is a more useful characteristic than dR/dT because the absolute resistance can be changed by changing the geometry of the resistor.) For resistor applications, some alloys have TCRs in the parts per million per xc2x0 C. range. However, other materials have much higher TCRs and can be used as temperature sensors, commonly called thermistors.
Desirable thermistor materials have constant (for ease of calibration) high (for sensitivity) TCRs over a wide operating temperature and reasonable resistivity values. However, some applications preferably have the highest possible TCR at the sacrifice of constancy and operating range. For example, bolometers detect absorbed radiation by producing an electrical signal in response to their increase in temperature. In a typical application, this increase is minute and constancy over a wide temperature range is unimportant. Bolometers may be optimized by maintaining them at a selected constant average ambient temperature and detecting fluctuations from varying radiation.
FIG. 1 is a chart summarizing the TCR and resistivity of some prior art materials and showing one example of the material of this invention in the upper center. A number of materials, thermistor multi-component oxides and GaAs or GaP based semiconductors in bulk form, have very high TCRs but also very high resistivities. These materials rely upon the excitation of electrons across a large thermal barrier for the high TCR. The large thermal barrier generally results in large resistivities which have the potential for non-ohmic contacts, and large 1/f noise levels. Prior art materials with lower resistances all have lower TCRs. Until now, for use in bolometers, for example, the best choice was V0x amorphous materials with a typical 0.02/xc2x0 C. (2%/xc2x0 C.) TCR.
These problems have received considerable attention for some time, and new materials with a higher TCR are always being sought. Various oxides have been investigated. For example, U.S. Pat. No 4,743,881, issued May 10, 1988 to Howng, discloses Laxe2x80x94Crxe2x80x94O based material, i.e., LaCrO3 with small amounts of Ti, Si, Mg, and/or Al substituted for Cr. This material is capable of operating in the range of 100 to 600xc2x0 C. These exhibit a TCR of about 2.5%/xc2x0 C. However, they are bulk devices made by calcining and sintering the oxides.
Materials with compositions similar to that of this invention are known. Laxe2x80x94Mnxe2x80x94O based materials, or more precisely (La1xe2x88x92xAx)MnO3 (A=Ca, Sr, Ba, Cd and Pb; 0 less than x  greater than 1), and Laxe2x80x94Coxe2x80x94O analogs were synthesized and extensively studied in early 1950s, see, for instance, G. H. Jonker and J. H. Van Santen, Physica, vol. 16 (1950) pp. 337-349 and pp. 599-600 and Physica, vol. 19 (1953) pp. 120-130. Jonker and Santen measured the ferromagnetic properties and conductivity of sintered powders as a function of temperature and composition. Recently, a great deal of interest was rekindled by the discovery of a xe2x80x9cgiant magnetoresistive effectxe2x80x9d in their thin film forms, see, for example, K. Chahara et al. Appl. Phys. Lett., vol 63 (1993) pp. 1990-1993 (reporting on Laxe2x80x94Caxe2x80x94Mnxe2x80x94O ion beam sputtered films on MgO substrates), R. Von Helmolt et al., Phys. Rev. Lett., vol. 71 (1993) pp.2331-2333 (reporting on Laxe2x80x94Caxe2x80x94Mnxe2x80x94O off-axis laser deposited films on SrTiO3), and S. Jin et al., Science, vol. 264 (1994) pp.413-415 (reporting on Laxe2x80x94Caxe2x80x94Mnxe2x80x94O films on LaAlO2 made via laser ablation of powders).
All these powders and films undergo a magnetic phase transition determined by the composition. Below the transition, they behave as metallic conductors but near the transition, the resistance increases sharply and then falls off almost as sharply above the transition temperature where they behave as semiconductors. In the presence of an external magnetic field, the resistance at all temperatures is reduced. For film materials, this decrease can be greater than a thousand-fold on application of a 6T magnetic field. Jin, et al, postulated that such giant magnetoresistance effects should lead to a variety of technical applications, but their results were obtained with transition temperatures of 77xc2x0 C. U.S. Pat. No. 5,487,356, issued Jan. 30, 1996 to Li (one of the present inventors) et al., incorporated herein by reference, discloses a metal oxide chemical vapor deposition (MOCVD) method of making giant magnetoresistive material of (La1xe2x88x92xAx)MnO3 (A=Ca, Sr, Ba, and Mg) with good magnetoresistance effect results at 270 K. However, there are no reports of any investigations of the TCR or use as temperature sensing materials, let alone optimization of the material composition for such use. This may be, in part, because the TCR is highly non-linear and only large within a few degrees of the transition temperature.
Even though it is known that a sharp resistance peak indicates the possibility of a ferromagnetic phase transition, the details are not usually disclosed. An exception is U.S. Pat. No. 5,538,800, issued Jul. 23, 1996 to Li et al., which discloses a polycrystalline material having a very high magnetoresistance ratio of 10,000% in a 6T magnetic field at about 140xc2x0 K. Although not discussed, the TCR can be deduced from a figure as about 15%/xc2x0 C., but this is also at about the same low temperature. When making a magnetic field sensor, a high TCR is a disadvantage because the device temperature must be held constant in order to accurately measure the magnetic field effects. A device structure to compensate the TCR was disclosed in U.S. Pat. No. 5,563,331, issued Oct. 8, 1996 to Von Helmolt. Therein, compositions with room temperature TCRs (deduced from figures) in the range of about 2-4%/xc2x0 C. are illustrated. The invention proposes a method of compensating for the TCR of a magnetoresistive sensor by using two layers of different-composition magnetically sensitive material with a low correlation between their TCRs.
All of these materials are based on a nominal LaMnOz composition with partial substitutions for La and Mn and having a perovskite-like structure. It is known that the perovskite structure is necessary in order to produce a ferromagnetic material. z is nominally 3, but can be in the range of about 2 to 3.5. A decrease from z=3 should occur if, for example, a divalent atom is substituted for the trivalent La. However, there is some uncertainty because some of the Mn atoms, which nominally has a valence of +3, can have a valence of +4 causing an increase in z or a valence of +2 causing a decrease in z. Moreover, there are usually oxygen deficiencies. It is known that in order to exhibit a ferromagnetic effect, there must be some Mn, about 30%, with a +4 valence. For instance, a material with an exact composition LaMnO3 is not ferromagnetic. Since considerable effort is required to determine the exact value of z and the amount of Mn with different valences for each composition, those skilled in the art understand that z has a range of values and often characterize the materials using nominal compositions in conjunction with physical properties.
One application where high a TCR is most useful is in bolometers where high sensitivity is desirable. U.S. Pat. No. 5,450,053, issued Sep. 12, 1995 to Wood, discloses a monolithic integrated focal plane sensitive to both mm-waves (typically 94 GHz) and IR radiation (typically 3-5 and 8-12 micron) constructed on a silicon wafer by selective anisotropic etching to fabricate microbolometer radiation sensors in a linear or two-dimensional array. Each microbolometer is constructed with a thin dielectric plate of amorphous silicon nitride or silicon dioxide attached to the silicon substrate and cantilevered over a groove in the substrate, as illustrated in FIG. 2. Vanadium oxide (VOx) is deposited on the dielectric and a radiation absorbing film deposited on the vanadium oxide. The VOx material has a TCR in the range of 1-4%/xc2x0 C. so that when heated by radiation its resistance will change. Since, in this application, the resistance change is very small, for maximum sensitivity, in order to overcome noise, it is desirable that the TCR is as high as possible. It is also desirable that the thermal mass and thermal conductivity to the substrate are as low as possible. These are attempted by using thin films and the cantilever structure. It is further desirable that the absolute resistance is compatible with readout circuitry, e.g., in the range of 100 to 100,000 ohms. Starting with material of reasonable resistivity, this can be obtained by using thin films in a serpentine configuration, if necessary. Another desirable characteristic is that the high TCR occurs near room temperature. However, these bolometers are typically cooled with a thermoelectric cooler so that somewhat lower temperatures, e.g., down to about 250 degrees K, can be used at the expense of some power for the cooler. The citation of this patent is provided as background to show that it is known how to make a microbolometer using thin films of TCR material and is not deemed as prior art to the invention disclosed in this application.
Accordingly, the main object of the invention is to provide materials which have the highest temperature coefficient of resistance (TCR) in a useful range of temperatures, preferably close to room temperature, so that sensitive temperature sensors can be made. A further object is to provide materials which could be produced in the form of a thin film so that sensitive radiation detectors, particularly of the multi-element or mosaic type could be produced. Still more objects are to provide materials which have reasonable resistivities so that practical value resistors can be formed and ones to which it is easy to make electrical contact with metals.
All of these objectives and more have been met with a temperature sensor comprising an oxide having a perovskite-like crystal structure which undergoes a ferromagnetic phase transition. In one embodiment, the oxide has a chemical formula LawAxByMnOz where
A is a divalent atom selected from an alkaline earth metal, or Mn or Pb,
B is a divalent atom selected from an alkaline earth metal, or Mn, or Pb, and
w+x+y=1, w, x, and y are in the range of 0 to 1, and z is in the range of 2 to 3.5.
By selecting various combinations of constituents, A and B, in varying amounts, the TCR can be maximized or the temperature at which the maximum TCR occurs can be adjusted. Examples are disclosed having a TCR greater than 5%/xc2x0 C. at a range of temperatures above xe2x88x9233xc2x0 C.
In one embodiment, the oxide is produced as an epitaxial thin film on a substrate. A variety of substrates can be used including single crystals whose crystal lattice size is matched to the crystal lattice size of the oxide. Alternately, single crystals with non-matched lattice sizes or even amorphous substrates can be used if suitable buffer layers such as YSZ, CeO2, or YBCO are used. It was discovered that the optimum oxide thin film is biaxially textured with a (100) crystal orientation and that suitable buffer layers promote the growth of such films. It was further discovered that annealing at 900xc2x0 C. in an oxygen atmosphere promotes oxygen stoichiometry. One sample showed and TCR of 15%/xc2x0 C. at xe2x88x9220xc2x0 C. This is the highest TCR known to us near room temperature.
The preferred method of making the invention uses the well known metal oxide chemical vapor deposition (MOCVD) process with suitable precursor chemicals. However, other methods can be used for some of the processing steps. When producing biaxially textured (100) YSZ buffer layers, it is believed that the most suitable method is the ion beam assisted deposition process to produce biaxially oriented single crystals or polycrystals. It is thought that this process would be particularly useful for producing such a film on an amorphous dielectric such as silicon nitride which is used in microbolometers as a an electrically and thermally isolating support structure for existing radiation sensor materials whose resistance changes due to heating by absorbed radiation. After the YSZ layer is formed, the MOCVD process can be used to produce buffer layers and the oxide temperature sensitive material. Using these methods, the films can be easily very thin to produce very low thermal mass sensors. Fortunately, it was found that thin films still had a reasonable resistivity and it was easy to make contacts with pressed metal or evaporated silver.